Railroad bridges are example structures which are necessary to monitor through occasional displacement measurements. There are presently about 140,000 miles of rail track across the United States. On average, there is presently one bridge for about every 1.4 miles of track. A bridge that is not performing adequately can result in train derailment, posing a very serious hazard to life and property. Furthermore, inadequately performing bridges can slow the advisable speed of trains, which reduces rail network capacity and increases rail delivery times.
Railroads maintain the quality of their bridges to guarantee the reliability and safety of their operations. Railroad bridge managers may inspect bridges annually. Bridge condition decay can be related to bridge displacements under train loads. If railroad bridge inspectors report large bridge displacements, railroad managers will limit the speed of trains crossing them to reduce these displacements and to avoid unsafe operations.
In general, railroad bridge managers currently observe bridge vibrations under trains visually, and estimate displacements qualitatively. In some circumstances, railroad bridge managers may also use Linear Variable Displacement Transducers (LVDTs) or accelerometers to measure bridge displacements. However, LVDT and accelerometer measurements are expensive, and at times not even feasible. Both LVDTs and accelerometers are contact devices, and their installation involves large amount of labor-intensive tasks at difficult to access locations.
It will be appreciated that, like railroad bridges, other structures may also benefit from occasional displacement measurements. For example, road bridges may be beneficially measured for similar reasons as rail bridges. Buildings, antennas, dams, and other structures may be beneficially measured during seismic events, or other events such as floods, extreme heat or cold, or high winds. For at least these reasons, there is a need for robust, flexible, effective, low cost, and easily deployed technologies for structural displacement measurement.